BA can issue boarding pass for HKG-BKK as well as check the baggage through. That is plenty of connection time, even with the delays out of LHR. I've made connections with 20 min at HKG (obviously not legal for MCT, but CX works magic for connections, it is maybe the most efficient airport in the world for connections on CX). If this were a CX/CX connection you would even be met at the gate and escorted to the connection, not sure if CX do that for BA arrivals.